US2728662A

US2728662A - Method of preparing photographic emulsions

Info

Publication number: US2728662A
Application number: US303394A
Authority: US
Inventors: Henry C Yutzy; Gordon F Frame
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1947-08-13
Filing date: 1952-08-08
Publication date: 1955-12-27
Anticipated expiration: 1972-12-27

Description

- United States Patent O METHOD OF PREPARING PHOTOGRAPHIC EMULSIONS Henry C. Yutzy and Gordon F. Frame, Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey No Drawing. Original application August 13, 1947, Se-

rial No. 768,475. Divided and this application August 8, 1952, Serial No. 303,394

13 Claims. c1. 95-7 This invention relates to the preparation of photographic emulsions having silver halide as the sensitive element in which the silver halide is prepared in dispersed form in gelatin which gelatin is then converted into a gelatin derivative whereby the thus-prepared gelatin derivative-silver halide dispersion may be coagulated by pH adjustment to form granules of silver halidegelatin derivative. This invention also includes the emulsions obtained by redispersing the so-obtained silver halide-gelatin derivative granules, using gelatin or gelatin derivative as the vehicle therefor.

This application is a division of our application No. 768,475, filed August 13, 194-7, now U. S. Patent No. 2,614,928.

The most common method of preparing silver halide dispersions is by reacting a water-soluble salt, such as silver nitrate, with a water-soluble halide, such as potassium bromide, in an aqueous solution of a peptizing agent. The dispersion of silver halide thus formed contains water-soluble salts as a byproduct of the double decomposition reaction. In processes of making silver halide dispersions using gelatin as the peptizer, the emulsion maker has, in the past, been limited in the scope of his operations by the physical properties peculiar to gelatin. These physical properties have necessitated the use of concentrations of silver halide and of gelatin sometimes, which are not the most desirable for photographic products. For instance, emulsions of high concentration of silver halide have been diflicult to prepare. Limitations have been imposed on the emulsion maker regarding the conditions of dilution during the emulsion making since emulsions of very high water content are difficult to coat and dry. Often the technician has been obliged to limit the minimum concentration of gelatin in the washing operation in which soluble salts are removed.

It has been considered desirable in the case of photographic emulsions, particularly those to be coated on water-impermeable supports, such as film base, that the soluble salts present in the emulsion be reduced in concentration. It has been the practice previously, where gelatin has been used as the protective colloid, to remove the soluble materials therefrom by setting the gel dispersion by means of chilling, noodling the so-set dispersion and washing the noodles in cold water. Under such conditions, a minimum gelatin concentration of approximately 4% is useful under practical conditions, and this limitation has hampered the operations of the emulsion maker in his preparation of emulsions by this method.

One of the objects of our inventionis to prepare dispersions of silver halides from which water-soluble byproducts may be readily removed. Another object of our invention is to provide a convenient practical method of preparing washed silver halide dispersions in which the dispersing colloid need be but a small percentage of the total composition. A further object of our invention is to provide a method of washing silver halide dispersions in which substantially all of the liquid may 2,728,662 Patented Dec. 27, 1955 be removed, thus making for complete removal of the water-soluble impurities therein. Other objects of our invention will appear herein.

Our invention is based upon the discovery that gelatin derivatives may be prepared from gelatin employed as a peptizer for preparing dispersions of silver halide and that dispersions of silver halide in those derivatives are acid coagulable and will form silver halide-gelatin derivative granules when the dispersion is adjusted to the proper pH. The method of our invention avoids the limitations which have previously characterized the prepara-. tion of washed silver halide dispersions and is valuable for expanding the possible conditions for the manufacture of photographic silver halide emulsions of different speeds, contrast and other photographic characteristics. In addition, our invention offers a simple means of preparing emulsions of high silver concentration, which is of value in the manufacture of photographic film bearing large quantities of silver per unit area.

Gelatin derivatives are prepared by reacting gelatin with a reagent, as set out hereinafter, at a pH of 8-11 or more, imparted by the addition of a suitable base, such as sodium hydroxide, preferably at a temperature of 20-60" C. When the preparation (at the gelatin derivative is carried out at a pH in the lower part of the range given, any suitable base may be used, such as ammonium hydroxide and sodium carbonate, as well as sodium hydroxide and the corresponding potassium compounds. In the upper part of the range the use of the stronger base, such as sodium or potassium hydroxide, is preferred, as a pH of 11 or more is attained with the use of moderate amounts of base. The reaction between the gelatin and the reagent employed therewith takes place within a very few minutes, and the silver halide dispersion takes on the character that it may be coagulated, forming silver halide-gelatin derivative granules upon acidulation of the mass. The insoluble materials which results consists of silver halide grains enveloped by gelatin derivative, and may be washed free of salts or any other water-soluble materials which may be present. As these granules may be completely separated from the liquid with which they are in contact, merely decanting the liquid from the granules removes substantially all of the water-soluble salt therefrom. However, still more effective removal of the water-soluble salts may be obtained by redispersing the granules, such as in water having a pH in which theywill disperse, and again coagulating and removing the liquid from the granules thus formed a second time. The coagulation of the silver halide dispersion in gelatin derivative is effected by acidifying the mass to a pH within the range of 3-4.5, whereby granules of silver halide and gelatin derivative will separate out. With a dispersion having a low salt concentration, the coagulum is best obtained in the upper part of the pH range given.

In carrying out our invention, the silver halide is prepared in an aqueous solution of gelatin as the peptizer, and the gelatin thus present is then converted into gelatin derivative prior to acidifying and washing by imparting a pH of 8-11 to the dispersion and then reacting the gelatin with the particular reagent employed. The silver halide maybe prepared by any of the normal methods for the preparation of silver halide dispersions, such as, for example, by introducing a stream of an aqueous solution of silver nitrate and a stream of an aqueous solution of an alkali metal halide, such as potassium bromide, into a vigorously agitated solution of gelatin. Instead, however, the gelatin solution can be combined with one of the reactants, and the other reactant may be introduced therein with vigorous stirring. After the silver halide has been formed, the gelatin has been converted into gelatin derivative and any ripening operations considered advisable have been carried out,

acid is added to the dispersion to lower the pH to a coagulating pH within the range of 3-4.5. The silver halide-gelatin derivative dispersion forms granules and quickly settles from the liquid portion of the mass. The coagulum thus formed may be separated from the liquid by any one of several techniques. In the simplest form, this supernatant mother liquor may be removed from the receptacle in which the mass is contained by means of a siphon, and the coagulum may then be rinsed with water to remove excess insoluble salts therefrom. The coagulum, after this washing operation, consists of silver halide, the gelatin derivative, any other water-insoluble materials which may be present, and a small amount of residual soluble salt dissolved in any water trapped in the coagulum.

For some types of emulsions this coagulum may be used directly without further washing, for instance, in

the preparation of emulsions for the coating of paper f or other permeable supports. Also, it may be employed for the coating of film or glass plates, but in that case it may be ordinarily preferred to reduce further the content of soluble salts which may be present by any one of several methods. For instance, the coagulum may be rinsed by treatment with cool water, preferably with the pH adjusted to the range of the isoelectric point of the gelatin derivative. The number of rinses which is desirable to reduce the salt content may be determined by experiment, but usually in this method 1 to 4 changes are suflicient. As an alternative washing method, the coagulum may be redispersed in Water at an elevated temperature using water and a small amount of alkaline material, such as sodium hydroxide or ammonium hydroxide, so that the pH of the mixture is of the order of 6 or higher. The temperature of the solution is raised to approximately 40 C., and the whole is stirred for a few minutes to effect redispersion of the coagulum. The gelatin derivative-silver halide grains may then again be precipitated by the addition of an appropriate acid solution to reduce the pH to the coagulation point of the derivative. The coagulum will again settle and the separation from the mother liquor is efiected as before. This redispersion and coagulation may be repeated as many times as is necessary, but for normal purposes ordinarily once is satisfactory to obtain a substantially salt-free material. As a third method of washing, the coagulum may be redispersed in water at a pH below 3.0 so as to disperse the silver halide-gelatin derivative grains. The low pH may be obtained by the addition of a quantity of an appropriate acid, such as sulfuric acid. Recoagulation may then be effected by the addition of a suitable base, such as ammonium hydroxide, to raise the pH of the solution to the coagulation point of the derivative. The coagulum will settle and the separation from the supernatant liquor is carried out as before.

The coagulum from which the water-soluble salts have been removed, either by decanting or by one of the 3 general methods outlined above, may then be redispersed to form a photographic emulsion suitable for the subsequent finishing and coating operations by mixing the coagulum with the required quantity of water, the vehicle to be employed for the silver halide, such as gelatin, and a base to raise the pH of the mass above that at which coagulation occurs. If desired, gelatin derivative, for instance, may be employed as the vehicle in the preparation of the photographic emulsion. The mixture of coagulum, water, vehicle and any other desired compounds is stirred at approximately 40 C. for a time sufficient to eifect a complete dispersal of the coagulum therein which usually requires 10 to minutes. The emulsion thus obtained has properties comparable to those of the best types of washed emulsions, and responds readily to chemical sensitization or after-ripening, and optical sensitization of the types employed upon photographic emulsions generally. The emulsions prepared in accordance with our invention may be coated onto a support using the techniques which are standard in the emulsion coating art.

In carrying out the method in accordance with our invention, the silver halide is first prepared in normal gelatin solution according to standard techniques for the preparation of the silver halide, and any ripening which may be desired is carried out. The pH of the mass is adjusted to a value of approximately 8 to 11 or more with a suitable base, for instance, sodium hydroxide or ammonium hydroxide, and the reagent for formation of the gelatin derivative is added in suitable physical form, usually dissolved in a solvent. After a suitable time has elapsed for the reaction between the reagent and the gelatin (5 minutes is ordinarily adequate), the pH of the mass may then be lowered to the coagulation region. We have found that gelatin derivatives, prepared by reacting standard photographic gelatins under conditions of elevated pH with aromatic sulfonyl chlorides, carboxylic acid chlorides, carboxylic acid anhydrides, aromatic isocyanates or 1,4-diketones in which derivatives of silver halide are suspended, lose their peptizing properties when a pH Within the range of 3-4.5 is imparted thereto. Upon adjusting the dispersion of silver halide in the gelatin derivative to a coagulating pH, the silver halide-gelatin derivative dispersion forms granules almost instantly, particularly where electrolyte is present in the suspension. In the second coagulation as referred to above, the salt concentration is considerably lower, and, in that case, it is ordinarily desirable to adjust the pH within a range of 3.8 to 4.2 to get the best coagulation of a silver halide dispersion.

For the acid which is to be employed for reducing the pH to the coagulation point, any acid may be employed which will give the desired pH in the mass and which will not deleteriously affect the photographic material. Dilute aqueous sulfuric acid has been found to be particularly useful for this purpose. Nevertheless, other acids, such as acetic acid or phosphoric acid, may be employed if sulfuric acid is not readily available.

In the preparation of gelatin derivatives, the gelatin in the aqueous dispersion of silver halide is treated with a solution of the reagent in a suitable solvent which is water miscible. Some solvents which are useful in this connection are acetone, dioxane, isopropyl alcohol, ethyl alcohol and the like. Some of the compounds which are suitable for use in converting the gelatin to a gelatin derivative in accordance with the procedure of our invention are as follows:

quin- Carboxylic acid chlorides: Phthalyl chloride p-Nitrobenzoyl chloride Benzoyl chloride Ethyl chlorocarbonate Furoyl chloride Methacrylyl chloride Acid anhydride:

Phthalic anhydride Benzoic anhydride Succinic anhydride Maleic anhydride Isatoic anhydride Methacrylic anhydride Isocyanates:

Phenyl isocyanate p-Bromophenyl isocyanate p-Chlorophenyl isocyanate p-Tolyl isocyanate p-Nitrophenyl isocyanate a-Naththyl isocyanate p-Naphthyl isocyanate 1,4,-diketones:

Acetonyl acetone Dimethyl acetonyl acetone Diethyl diacetyl succinate It is apparent that one advantage of our method is that the concentration of silver halide in the final emulsion may be adjusted to any desired value without reference to the quantities of water, silver halide and gelatin employed in the initial reaction for the preparation of the silver halide dispersion. Thus, concentrated emulsions may be readily prepared. We have also found that, by our technique, dried emulsions may be prepared for storage and shipment without any necessity of evaporation of water or reduction of the water content of the dispersed silver halide and without the necessity of using concentrated salt solutions or organic solvents. We have found that the coagulum prepared by the use of gelatin derivatives is especially suitable for the preparation of dried emulsions, since it may be obtained in a state nearly free of excess water, and therefore the drying operation is simplified and rendered less expensive. The coagulum may be obtained in a suitable physical form for the purpose, and the dried emulsion may be readily redispersed in water containing appropriate alkali at any time. In this connection we have found that washed coagulum prepared from the dicarboxylic acid anhydride derivatives of gelatin, such as that produced from phthalic anhydride, is especially useful. These derivatives are described and claimed in our Patent No. 2,525,753.

We have also found that the coagulation operation may be carried out, either after the preparation of silver halide to remove water-soluble salts or after a heat treatment with appropriate chemical sensitizers, so that coagulated silver halide grains are obtained before or after having been brought to optimum photographic speed. These grains may be dried at either stage in the cycle Without harming their properties when finally made up in the emulsion. The following examples illustrate our invention:

Example 1 100 grams of silver nitrate were dissolved in 900 cc. of distilled water at 25 C. 80 grams of potassium bromide, 1 gram of potassium iodide and 25 grams of gelatin were dissolved in another 900 cc. of distilled water. The temperature was adjusted to 50 C. and the silver nitrate solution was added to the bromide-gelatin solution over a period of approximately 10 minutes, accompanied by vigorous stirring of the bromide-gelatin solution, while maintaining the temperature at 50 C. The mass was then stirred for 10 minutes at 50 C. and was then cooled to 40 C. The pH of the so-obtained silver halide dis persion was adjusted to 9.5 with dilute ammonium hydroxide, and then 3 cc. of benzene sulfonyl chloride in 40 cc. of dry acetone was added to the dispersion. The mass was stirred at 40 C. for 5 minutes, whereupon the benzene sulfonyl chloride derivative of gelatin was formed in the mass. Dilute aqueous sulfuric acid was then added, whereupon the pH was lowered to approximately 3. Silver halide-gelatin derivative granules formed and quickly dropped to the bottom of the container. The mother liquor was decanted ofi and the silver halide grains were washed twice with distilled water at 20 C. There was then mixed with the silver halide grains 150 grams of gelatin in solution in 1500 cc. of distilled water, and the mass was stirred at 40 C. Ammonia was added to bring the pH up to 6.5, and stirring was continued until all of the solids were completely dispersed in the liquid. The resulting emulsion was then processed and coated, as is usual for photographic emulsions.

Example 2 80 grams of potassium bromide, 1 gram of potassium iodide and grams of gelatin were dissolved in 900 cc. of distilled water at 50 C. There was then added to this solution over a period of 10 minutes a solution of 100 grams of silver nitrate in 900 cc. of distilled water, also at 50 C. The resulting dispersion was maintained at 50 C. for 10 minutes. The mass was then cooled to 40 C. The pH of the dispersion was adjusted to approximately 9 with dilute aqueous ammonium hydroxide, and a solution of 2.5 grams of phenyl isocyanate in 30 cc. of dry acetone was added. The mass was stirred for approximately 5 minutes while maintaining the pH thereof at about 9.5 by periodical additions of dilute ammonium hydroxide. The gelatin was converted thereby to the phenyl isocyanate derivative thereof. The pH of the dispersion was then adjusted to 3 by the addition of dilute sulfuric acid. Silver halide granules quickly formed and the clear supernatant liquid was decanted. The coagulum thus formed was dispersed in an equal volume of distilled water at 40 C. at a pH of no more than 3. The pH was then raised to 3.842 with dilute ammonium hydroxide, whereupon the coagulum again formed, settled, and the liquid was decanted ofi. A photographic emulsion was made up using this coagulum with the desired amount of gelatin and Water. The photographic emulsion thus obtained upon testing was found to have normal sensitometric characteristics on finishing.

, Example 3 A silver bromoiodide dispersion was prepared containing 1 mol of silver halide peptized in a gelatin solution consisting of 27 grams of photographic gelatin dissolved in 4500 cc. of distilled water. A 2250 cc. portion of the emulsion was taken, and the temperature thereof was adjusted to 40 C. 24 cc. of 3 normal ammonium hydroxide were added raising the pH to 10.0. 1.4 cc. of phthalyl chloride, which had been dissolved in 15 cc. of acetone, were added and the mass was stirred for 5 minutes, during which time the pH gradually dropped to 8.6. The pH was then dropped to 3.0 with 2 normal sulfuric acid. The silver halide-gelatin derivative granules formed which separated from the liquid portion of the emulsion. The liquid was then decanted 01f, the granules were washed with distilled water, additional distilled water was added and the grains were redispersed at a pH of 6.0 at a temperature of 40 C. A washed silver halide photographic emulsion is prepared from this dispersion by mixing it with the desired amount of gelatin or aqueous gelatin in the usual manner.

Example 4 the pH had dropped to 9.2. The pH of the mass was then adjusted to 3.0 by adding 2 normal sulfuric acid, whereupon silver halide-gelatin derivative granules formed and separated from the liquid. The grains were separated from the liquid, washed and redispersed in distilled water at a pH of 6.0 and a temperature of 40 C. A washed silver halide photographic emulsion is prepared from this dispersion by mixing it with the desired amount of gelatin or aqueous gelatin in the usual manner.

Example A silver bromoiodide dispersion was prepared of 1 mol of silver halide dispersed in a gelatin solution consisting of 27 grams of photographic gelatin in 4500 cc. of distilled water. 2250 cc. of this dispersion were taken. The temperature was raised to 40 C. and the pH was adjusted to 9.7 by adding 3 normal ammonium hydroxide thereto. 1.4 grams of naphthalene-beta-sulfonyl chloride, dissolved in a mixture of 15 cc. of isopropyl alcohol andv 15 cc. of acetone were added and the mass was stirred for minutes at 40 C., dropping the pH to 9.0. The pH was then adjusted to 3.0 with 2 normal sulfuric acid forming silver halide-gelatin derivative granules which settled out. The liquid was decanted oil? and, after Washing the grains several times in distilled water, they were redispersed in water at a temperature of 40 C. and a pH of 7.0. A washed silver halide-gelatin derivative photographic emulsion is prepared from this dispersion by mixing it with the desired amount of gelatin or aqueous gelatin in the usual manner.

Example 6 A silver bromoiodide dispersion was prepared of 1 mol of silver halide in a solution of 27 grams of photographic gelatin in 4500 cc. of distilled water. 2250 cc. of this dispersion were taken and the temperature thereof was raised to 40 C. The pH thereof was adjusted to 9.5 by adding ammonium hydroxide. 1.6 grams of alphanaphthyl-isocyanate dissolved in cc. of dry acetone were added and the mass was stirred for 10 minutes at 40 C., which dropped the pH to 9.3. The pH was then adjusted to 3.0 by adding 2 normal sulfuric acid, whereupon silver halide-gelatin derivative granules were formed and settled out. The grains were rinsed 3 times with cold water, whereupon they were mixed with 118 grams of photographic gelatin and the weight was adjusted to 5 /2 lbs. by adding distilled water. The mass was dispersed at 40 C. at a pH of 7. After coagulating and redispersing, the pH was adjusted to 6.0, and 3 grams of potassium chloride were added thereto. A sample thereof was heated for minutes at 100 C., cooled to C. and coated out on a glass plate. After drying, the coated plate was exposed and processed, and the emulsion thus obtained was found to have acceptable photographic properties.

Example 7 Solution A was prepared consisting of grams of photographic gelatin, 282 grams of potassium bromide and 2.6 grams of potassium iodide in 3000 cc. of distilled water at C. Solution C was prepared of 34-0 grams of silver nitrate in 3600 cc. of distilled water at 62 C. Solution C was added to solution A over a period of approximately 11 minutes with good mechanical stirring. The mass was then cooled to 40 C. and divided into 4 equal parts as follows:

Part 1.The pH of this portion was adjusted to 10.0 by adding 2.5 normal sodium hydroxide thereto. 1.25 grams of phthalic anhydride in solution in 15 cc. of dry acetone were slowly added to the mass and the pH was held at 10.0 by adding sodium hydroxide solution thereto from time to time. The mass was stirred for 10 minutes at 40 C., cooled to 35 C. and the pH thereof was then adjusted to 3.0 with 2 normal sulfuric acid. Granules formed therein and settled out, and the supernatant liquid was decanted off. Distilled water was added to the grains, and the mass was stirred at a temperature of 35 C. and a pH of 3.0 for 5 minutes, whereupon the grains dispersed in the water. The pH was adjusted to 4.0 atwhich point granules again formed and settled out, and the supernatant liquid was decanted off. 130 grams of photographic gelatin were then added, and the total weight was brought to 4 lbs. by adding distilled water. The mass was dispersed at 40 C. and a pH of 7 for 30 minutes. The pH was then adjusted to 6.0 and the emulsion was heated for 25 minutes at 60 C. A portion of the emulsion was coated on a cellulose acetate film support, and, after drying, a sample of this film coating was exposed on an intensity scale sensitometer (Kodak Ib) and processed for 3 minutes in developer D-l9.

Part 2.-T he procedure of Part 1 was repeated except that tie amount of phthalic anhydride employed was 2.5 grams in solution in 30 cc. of dry acetone.

Part 3.-The procedure of Part 1 was repeated except that 1.5 grams of maleic anhydride was employed instead of phthalic anhydride.

Part 4.The procedure of Part 2 was repeated except that maleic anhydride was employed instead of phthalic anhydride.

Upon exposure and processing, the various emulsions obtained were found to give the following sensitometric results:

Part N 0. Speed 10/i Gamma Fog We claim:

1. In a method of preparing washed silver halide dispersions, the steps which comprise mixing together a watersoluble silver salt and a water-soluble halide salt in an aqueous solution of gelatin, thereby forming a silver halide dispersion, treating the thus-formed dispersion with a gelatin derivative-forming reagent selected from the group consisting of the aromatic sulfonyl chlorides, the carboxylic acid chlorides, the carboxylic acid anhydrides, the aromatic isocyanates and the 1,4 diketones, at a pH of 8-11 whereby a silver halide-gelatin derivative dispersion is formed, coagulating this dispersion by adjusting to a coagulating pH within the range of 3-4.5 whereby silver halide-gelatin derivative granules are formed and separating the silver halide-gelatin derivative granules from the liquid portion of the mass.

2. A method of preparing aeid-coagulable silver halide dispersions which comprises mixing together a watersoluble silver salt and a water-soluble halide salt in an aqueous solution of gelatin, and subsequently treating the dispersion with an aromatic sulfonyl chloride at a pH of 8-11 whereby a silver halide-gelatin derivative dispersion is formed.

3. A method of preparing acid-coagulable silver halide dispersions which comprises mixing together a watersoluble silver salt and a water-soluble halide salt in an aqueous solution of gelatin, and subsequently treating the dispersion with a carboxylic acid chloride at a pH of 8-11 whereby a silver halide-gelatin derivative dispersion is formed.

4. A method of preparing acid-coagulable silver halide dispersions which comprises mixing together a watersoluble silver salt and a water-soluble halide salt in an aqueous solution of gelatin, and subsequently treating the dispersion with a carboxylic acid anhydride at a pH of 8-11 whereby a silver halide-gelatin derivative dispersion is formed.

5. A method of preparing acid-coagulable silver halide dispersions which comprises mixing together a watersoluble silver salt and a water-soluble halide salt in an aqueous solution of gelatin, and subsequently treating the dispersion with an aromatic isocyanate at a pH of 8-11 whereby a silver halide-gelatin derivative dispersion is formed.

6. A method of preparing acid-coagulable silver halide dispersions which comprises mixing together a watersoluble silver salt and a water-soluble halide salt in an aqueous solution of gelatin whereby a silver halide dispersion is formed, and subsequently treating the dispersion with benzene sulfonyl chloride at a pH of 8-11 whereby a silver halide-gelatin derivative dispersion is formed.

7. A method of preparing acid-coagulable silver halide dispersions which comprises mixing together a watersoluble silver salt and a water-soluble halide salt in an aqueous solution of gelatin whereby a silver halide dispersion is formed, and subsequently treating the dispersion with phenyl isocyanate at a pH of 8ll whereby i a silver halide-gelatin derivative dispersion is formed.

8. A method of preparing acid-coagulable silver halide dispersions which comprises mixing together a watersoluble silver salt and a Water-soluble halide salt in an aqueous solution of gelatin whereby a silver halide dispersion is formed, and subsequently treating the dispersion with phthalic anhydride at a pH of 8-11 whereby a silver halide-gelatin derivative dispersion is formed.

9. A method of preparing acid-coagulable silver halide dispersions which comprises mixing together a watersoluble silver salt and a Water-soluble halide salt in an aqueous solution of gelatin whereby a silver halide dispersion is formed, and subsequently treating the dispersion with maleic anhydride at a pH of 8l1 whereby a silver halide-gelatin derivative dispersion is formed.

10. A method of preparing acid-coagulable silver halide dispersions which comprises mixing together a water-soluble silver salt and a water-soluble halide salt in an aqueous solution of gelatin whereby a silver halide dispersion is formed, and subsequently treating the dispersion with phthalyl chloride at a pH of 8-11 whereby a silver halide-gelatin derivative dispersion is formed.

11. In a method of preparing washed silver halide dispersions, the steps which comprise mixing together a water-soluble silver salt and a water-soluble halide salt in an aqueous solution of gelatin whereby a silver halide dispersion is formed, treating the dispersion with a reagent selected from the group consisting of the aromatic sulfonyl chlorides, the carboxylic acid chlorides, the carboxylic acid anhydrides, the aromatic isocyanates and the 1,4 diketones whereby a silver halide-gelatin derivative dispersion is formed, coagulating said dispersion by adjusting to a coagulating pH within the range of 3-4.5 whereby silver halide-gelatin derivative granules are formed, separating those granules from the liquid portion of the mass, redispersing the granules thus obtained in water at an elevated pH, followed by adjusting the pH of the solution to that which will coagulate the silver halide dispersion, and separating the silver halide-gelatin derivative granules formed thereby from the liquid portion of the mass.

12. In a method of preparing washed silver halide dispersions, the steps which comprise mixing together a water-soluble silver salt and a Water-soluble halide salt in an aqueous solution of gelatin, thereby forming a silver halide dispersion, treating the dispersion with a carboxylic acid anhydride at a pH of 8-11 whereby a silver halide-gelatin derivative dispersion is formed, coagulating the said dispersion by adjusting to a coagulating pH within the range of 3-4.5 whereby silver halidegelatin derivative granules are formed, separating those granules from the liquid portion or" the mass, redispersing those granules in water at an elevated pH, followed by adjusting the solution to a pH which will coagulate the silver halide dispersion, which pH is within the range of 3-4.5, and separating the thus-formed silver halidegelatin derivative granules from the liquid portion of the mass.

13. A method of preparing a photographic emulsion which comprises preparing a silver halide dispersion in an aqueous solution of gelatin, treating the dispersion thus formed with phthalic anhydride at a pH of 8-11 whereby a silver halide-gelatin derivative dispersion is formed, adding dilute aqueous acid to the dispersion so as to form a coagulum of silver halide granules, settling and decanting the supernatant liquid therefrom, and subsequently mixing the coagulurn with an aqueous solution of gelatin as a protective colloid for the silver halide whereby a gelatino-silver halide photosensitive emulsion is formed.

References Cited in the file of this patent UNITED STATES PATENTS 718,312 Cobenzl Jan. 13, 1903 1,844,716 Lambert Feb. 9, 1932 2,139,774 Sheppard et al. Dec. 13, 1938 2,142,311 Heidenhain Jan. 3, 1939 2,282,001 Russell et al. May 5, 1942 2,401,051 Crouse et al. May 28, 1946 2,489,341 Waller et a1 Nov. 29, 1949 FOREIGN PATENTS 479,419 Great Britain Feb. 4, 1938 537,256 Great Britain June 16, 1941 580,504 Great Britain Sept. 10, 1946 649,546 Great Britain Ian. 31, 1951

Claims

1. IN A METHOD OF PREPARING WASHED SILVER HALIDE DISPERSIONS, THE STEPS WHICH COMPRISE MIXING TOGETHER A WATERSOLUBLE SILVER SALT AND A WATER-SOLUBLE HALIDE SALT IN AN AQUEOUS SOLUTION OF GELAIN, THEREBY FORMING A SILVER HALIDE DISPERSION, TREATING THE THUS-FORMED DIDPERSION WITH A GELATIN DERIVATIBE-FORMING REAGENT SELECTED FROM THE GROUP CONSISTING OF THE ORAMATIC SULFONYL CHLORIDES, THE CARBOXYLIC ACID CHLORIDES, THE CARBOXYLIC ACID ANHYDRIDES, THE ARMATIC ISOCYANTES AND THE 1,4 DIKETONES, AT A PH OF 8-11 WHEREBY A SILVER HALIDE-GELATIN DERVATIVE DISPERSION IS FORMED, COAGULATING THIS DISPERSION BY ADJUSTING TO A GOAGULATING PH WITHIN THE RANGE OF 3-4.5 WHEREBY SILVER HALIDE-GELATIN DERIVATIVE GRANULES ARE FORMED AND SEPARATING THE SILVER HALIDE-GELATIN DERIVATIVE GRANULES FROM THE LIQUID PORTION OF THE MASS.